Drupal\Core\Field\FieldDefinition::getSchema() is broken

#2144327: Make all field types provide a schema() added a call to getFieldType which doesn't even exist. Then it tries to add an array to a NULL which fatals. There are, obviously, no tests. I haven't written any but now that migrate uses this, that's ample test... Migrate just does this:

      $schema = FieldDefinition::create($type)->getColumns();
      return $schema[$column];

Also, StringItem tries to get max_length by running $field_definition->getSetting('max_length') but at this moment $field_definition has no settings. If I patch up FieldDefinition::schema to do

      $this->setSettings($definition['settings']);
      $schema = $class::schema($this);

then it works but it's clearly wrong -- if someone tries to run just FieldDefinition::create('string')->getSetting('max_length') that wouldn't work. Not sure what to do here. TypeData is still very strange to me.